The use of synchronous rectifiers to improve power converter efficiency is well known in the art. Power converters employing synchronous rectifiers, however, are typically more complex than conventional power converters employing diode rectifiers. Synchronous rectifiers typically contain a plurality of rectifying switches. The greater complexity, therefore, stems from the generation of drive signals for driving control terminals of the rectifying switches. Many techniques for driving the control terminals are available in the art. The available techniques may be separated into two broad categories, namely, control driven and self driven.
Control driven techniques generally employ a semiconductor-based control circuit to generate and synchronize the drive signals. The control circuit may include, for example, a Pulse Width Modulation (PWM) control integrated circuit (such as the UC1842 manufactured by Unitrode of Merrimack, N.H.) or a MOSFET driver integrated circuit (such as the TPS2812 manufactured by Texas Instruments of Dallas, Tex.). The drive signals should be properly synchronized, either to power switches on a primary side of an isolation transformer, or to voltages present on secondary windings of the isolation transformer. Proper synchronization of the drive signals is critical, since excessive power dissipation or even destruction of the power converter may result if the rectifying switches are turned on (or off) at an inappropriate time or, alternatively, are turned on simultaneously.
Self driven synchronous rectifiers generally use the secondary windings of the isolation transformer to generate and to synchronize drive signals. One advantage of the self driven technique lies in its inherent simplicity. Another advantage is that the drive signals are self synchronized. Additional circuitry is usually not required to properly synchronize the drive signals with other portions of the power converter.
Examples of power converters employing self driven synchronous rectifiers may be found in U.S. Pat. Nos. 5,303,138 and 5,528,482, entitled, "Low loss Synchronous Rectifier for Application to Clamped Mode Power Converters," by Rozman. Additional examples may be found in U.S. Pat. No. 5,590,032, entitled, "Self Synchronized Drive Circuit for a Synchronous Rectifier in a Clamped-Mode Power Converter", by Bowman, et al.; U.S. Pat. No. 5,274,543, entitled "Zero-Voltage Switching Power Converter with Lossless Synchronous Rectifier Gate Drive," by Loftus; U.S. Pat. No. 5,434,768, "Fixed Frequency Converter Switching at Zero Voltage," by Jitaru, et al.; and U.S. Pat. No. 5,535,112, entitled "DC/DC Conversion Circuit," by Vasquez Lopez, et al. The aforementioned references are incorporated herein by reference.
A conventional active clamp self driven synchronous rectifier power converter typically includes a drive train on a primary side of an isolation transformer. The power converter further includes a synchronous rectifier, consisting of first and second rectifying switches, coupled to first and second secondary windings of the isolation transformer, respectively.
The drive train generally includes power switches coupled to a primary winding of the isolation transformer. The drive train further includes an active clamp circuit that limits a reset voltage across the primary and secondary windings of the isolation transformer during a transformer reset interval. Drive signals to the rectifying switches may, therefore, be substantially free of dead time. By maintaining a substantially constant drive signal to one rectifying switch during the transformer reset interval, the active clamp circuit may increase power converter efficiency. In practice, however, the reset voltage may not be constant, due to variations in component selection and construction.
One disadvantage of the self driven synchronous rectifier power converter is that a potential of the drive signals is substantially proportional to an output voltage of the power converter. Therefore, in applications wherein a low output voltage is required (e.g., an output voltage of 3.3 V or less), the potential may be inadequate to drive the rectifying switches. An additional drive winding on the secondary side of the isolation transformer has been proposed to positively increase the potential of the drive signals. The additional drive winding, however, may cause the potential to be negative as well as positive (with respect to a source voltage of the rectifying switch). Since driving the rectifying switch negative may cause significant power loss, the benefits of the increased potential is mitigated.
Accordingly, what is needed in the art is a drive circuit for a self driven synchronous rectifier power converter that provides the benefits of an additional drive winding (e.g., increased positive potential of the drive signal) that overcomes the deficiencies in the prior art.